1. Field of the Invention
The present invention relates to a sheet conveying device, an image forming apparatus, and a method of determining whether multi-feed has occurred.
2. Description of the Related Art
Techniques of determining whether or not multi-feed of sheets has occurred (hereinafter, “multi-feed determination”) based on difference in amount (e.g., an amount of attenuation) of an ultrasonic wave between before and after passing through a sheet(s) being conveyed are conventionally known. Meanwhile, the difference in amount of the ultrasonic wave largely depends not only on the factor of the sheet but also on the factor other than the factor of the sheet. Accordingly, devices (devices that make multi-feed determination) undesirably vary from each other in the difference in amount of the ultrasonic wave, which leads to a false multi-feed determination.
Examples of the factor of the sheet include basic weight of the sheet. Examples of the factor other than the sheet include variation in timing of sending an ultrasonic wave by a sending sensor, variation in timing of receiving an ultrasonic wave by a receiving sensor, variation in arrangement of the sending sensor and the receiving sensor, and device-to-device environmental variation (e.g., variation in the temperature, the humidity, or the like).
A technique of adjusting an amplification factor of an amplifier circuit of a signal output by a receiving sensor receiving a ultrasonic wave is disclosed in, for example, Japanese Laid-open Patent Application No. 2006-312527. According to the technique disclosed in Japanese Laid-open Patent Application No. 2006-312527, device-to-device variation in the difference in amount of the ultrasonic wave can be adjusted, and occurrence of false multi-feed determination can be reduced.
However, such a conventional technique as that described above that adjusts variation in the difference in amount of the ultrasonic wave by using an amplification factor of an amplifier circuit is disadvantageous in that noise filtering characteristics of the amplifier circuit are also undesirably changed, undesirably causing oscillation of an operational amplifier or the like to occur. As a corollary to this, multi-feed determination will fail. For this reason, such a conventional technique as that described above is susceptible to improvement in accuracy of multi-feed determination.
In light of the foregoing, there is a need for a sheet conveying device, an image forming apparatus, and a multi-feed determination method with improved accuracy of multi-feed determination.